We have studied the interaction of molecular fluorine with a Si(100)2x1 surface. Three scattering channels were observed to be present in the interaction of low energy F2 with Si(100): unreactive scattering, two-atom adsorption and single atom abstraction. Single atom abstraction can be considered as the reverse of the Eley-Rideal mechanism, and our lab was the first to provide experimental verification of this novel gas-surface reaction mechanism. Two atom adsorption is a stepwise process which can be contrasted to the concerted process that characterizes classical dissociative chemisorption. The absolute probabilities of these three scattering channels were determined as a function of fluorine coverage. On the clean Si(100) surface, two atom adsorption is the dominant reaction channel (P₂=0.83±0.03) relative to that of single atom abstraction (P₁=0.13±0.03). The total reactivity of the surface decreases with coverage as the number of unoccupied reactive sites, identified to be the Si dangling bonds using He diffraction, decreases. However, the probability of single atom abstraction increases at the expense of two atom adsorption attaining a maximum (P1=0.35±0.08) at a coverage of 0.5 monolayers (ML). At 1 ML coverage, there are no unoccupied dangling bonds and the reaction with F₂ ceases. No etching is observed to occur.

A statistical model has been developed that well describes the coverage dependence of the reaction probabilities of F₂ with Si(100). The model is based on the premise that the two dissociative chemisorption mechanisms share a common initial step, F atom abstraction. The subsequent interaction, if any, of the complementary F atom with the surface determines if the overall result is single atom abstraction or two atom adsorption. The results are consistent with the orientation of the incident F₂ molecular axis with respect to the surface affecting the probability of single atom abstraction relative to two atom adsorption. A perpendicular approach favors single atom abstraction because the complementary F atom cannot interact with the surface, whereas a parallel approach allows the F atom to interact with the surface and adsorb. The fate of the complementary F atom is dependent on the occupancy of the site with which it interacts. The model is also based on the premise that the four distinguishable types of sites on the Si(100)(2x1) surface, based on the occupancy of the site itself and the complementary Si atom in the Si surface dimer, have different reactivities with F₂ and F atoms. The results show that the unoccupied sites on half-filled dimers are more reactive than those on empty dimers, which is consistent with an enhanced reactivity due to a loss of a stabilizing pi interaction between the two unoccupied dangling bonds on a dimer.

We have also studied the dynamics of xenon difluoride interacting with Si(100)2x1. Three analogous scattering channels to those observed with F₂ are also present in the interaction of XeF₂ with Si(100) at 250 K. Despite the observation that XeF₂ is able to etch Si, unlike F₂, He diffraction shows that the initial adsorption of fluorine upon exposure to XeF₂ occurs primarily at the dangling bonds. However, at a coverage around 1 ML, although there are no unoccupied dangling bonds on the well-ordered surface, XeF₂ continues to react with Si and the etch product SiF₄ is desorbed.